fix: avoid NPE when previewing images with redis cache

CacheServiceRedisImpl#getImgCache(String) passed the key straight to Redisson, which throws NullPointerException (map key can't be null) for a null key. Every single-image preview hits this path because compressFileKey is only set for images inside an archive, so with cache.type=redis the preview endpoint always returned HTTP 500. Return an empty list for empty keys, matching CacheServiceJDKImpl.

Fixes #793

server/src/main/java/cn/keking/service/cache/impl/CacheServiceRedisImpl.java

@@ -1,12 +1,14 @@
 package cn.keking.service.cache.impl;
 
 import cn.keking.service.cache.CacheService;
+import org.apache.commons.lang3.StringUtils;
 import org.redisson.api.RBlockingQueue;
 import org.redisson.api.RMapCache;
 import org.redisson.api.RedissonClient;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnExpression;
 import org.springframework.stereotype.Service;
 
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
 
@@ -68,6 +70,9 @@ public class CacheServiceRedisImpl implements CacheService {
 
     @Override
     public List<String> getImgCache(String key) {
+        if (StringUtils.isEmpty(key)) {
+            return new ArrayList<>();
+        }
         RMapCache<String, List<String>> convertedList = redissonClient.getMapCache(FILE_PREVIEW_IMGS_KEY);
         return convertedList.get(key);
     }

server/src/test/java/cn/keking/service/cache/impl/CacheServiceRedisImplTests.java

@@ -0,0 +1,65 @@
+package cn.keking.service.cache.impl;
+
+import cn.keking.service.cache.CacheService;
+import org.junit.jupiter.api.Test;
+import org.redisson.api.RMapCache;
+import org.redisson.api.RedissonClient;
+
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verifyNoInteractions;
+import static org.mockito.Mockito.when;
+
+/**
+ * CacheServiceRedisImpl 回归测试。
+ *
+ * 背景：cache.type=redis 时预览单张图片，compressFileKey 为 null，
+ * 修复前会把 null 直接传给 Redisson，抛出 NullPointerException: map key can't be null。
+ */
+class CacheServiceRedisImplTests {
+
+    @Test
+    void shouldReturnEmptyListWhenKeyIsNull() {
+        RedissonClient redissonClient = mock(RedissonClient.class);
+        CacheServiceRedisImpl cacheService = new CacheServiceRedisImpl(redissonClient);
+
+        List<String> result = cacheService.getImgCache(null);
+
+        assertNotNull(result);
+        assertTrue(result.isEmpty());
+        // 判空后不应再访问 Redis —— 修复前这里会调用 getMapCache 并抛出 NPE
+        verifyNoInteractions(redissonClient);
+    }
+
+    @Test
+    void shouldReturnEmptyListWhenKeyIsEmpty() {
+        RedissonClient redissonClient = mock(RedissonClient.class);
+        CacheServiceRedisImpl cacheService = new CacheServiceRedisImpl(redissonClient);
+
+        List<String> result = cacheService.getImgCache("");
+
+        assertNotNull(result);
+        assertTrue(result.isEmpty());
+        verifyNoInteractions(redissonClient);
+    }
+
+    @Test
+    void shouldReadFromRedisWhenKeyIsPresent() {
+        RedissonClient redissonClient = mock(RedissonClient.class);
+        @SuppressWarnings("unchecked")
+        RMapCache<String, List<String>> mapCache = mock(RMapCache.class);
+        when(redissonClient.<String, List<String>>getMapCache(CacheService.FILE_PREVIEW_IMGS_KEY))
+                .thenReturn(mapCache);
+        when(mapCache.get("zip-key")).thenReturn(List.of("a.png", "b.png"));
+
+        CacheServiceRedisImpl cacheService = new CacheServiceRedisImpl(redissonClient);
+
+        List<String> result = cacheService.getImgCache("zip-key");
+
+        assertEquals(List.of("a.png", "b.png"), result);
+    }
+}